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Truetone console pull

Yes, that type amplifier can be very transparent, mostly it's a function of output transformer quality. No, coupling cap value isn't overly critical, because they're in the feedback loop. Did you ever measure the amount of feedback being applied? It's essential to know that if you intend to optimize the amplifier further. Just compare the overall gain with and without the loop connected.

I agree with comments above by @gadget73 .

Jack
 
The question about coupling capacitors, wasn't about this specific amplifier, but about amplifiers in general. I'm happy with this one, I'm going to consider it finished. I was just curious if you had an amplifier without any kind of feedback circuit, which components would most affect the freq response.
 
With no feedback the cap values directly affect response, especially with multiple stages. The roll off points stack up and end up with a much steeper slope.

The transformers are the other big bottle neck.
 
Not an unreasonable figure. Datasheet at 300v plate and screen says 11 watts, but at 13 percent distortion. Looks like you're more like 280 based on the schematic so thats a drop. I believe this is fixed bias too, so with cathode bias it'll be a little lower as well. Datasheet is also plate power, figure 80-ish% transformer efficiency and knocking the output down enough that its at a sane distortion level and I can believe 6 watts of clean power into a speaker load.

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might not be a terrible idea to see what the transformer ratio is. It may be intended for something other than 8 ohms. A mismatch will cause power loss too.

I also suspect there might be a little more to be had if you bump the grid resistors on the 7868. Highly possible its the 12ax7 running out of drive, not the output stage.
 
What would you suggest bumping the 47k resistor to? If I did that, is that going to change the square waveform? Don't really need more power, and I like the way it sounds now. Only benefit I can see is easing the load on the 12AX7.
 
wlgrant6 said:
What would you suggest bumping the 47k resistor to? If I did that, is that going to change the square waveform? Don't really need more power, and I like the way it sounds now. Only benefit I can see is easing the load on the 12AX7.
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Datasheet allows for 1M in cathode bias, honestly a 470K wouldn't be crazy. Shouldn't bother the HF tuning any either. i suspect it will tighten up the bass a bit too.

Easy enough experiment, if you don't like it you can put it back or find some happy medium between 47k and 470k.
 
gadget73 said:
Datasheet allows for 1M in cathode bias, honestly a 470K wouldn't be crazy. Shouldn't bother the HF tuning any either. i suspect it will tighten up the bass a bit too.

Easy enough experiment, if you don't like it you can put it back or find some happy medium between 47k and 470k.
Click to expand...
I'll give that a try when I get a chance. Thanks gadget.
 
OK, replaced the 47K grid resistors with 470K resistors. Before I replaced them, I adjusted the signal generator for a 2 volt 10khz square wave signal on the scope. After changing the resistors, with everything still set up on the sig gen and the scope, I had 3 volts on the scope on the square wave, and the waveform wasn't appreciably changed. Looks like overall power was increased, however it still starts clipping with a 1khz signal at about 6 watts. Looks reasonably flat with a quick check from 1khz to 20. Sound wise, still sounds good, maybe bass a bit cleaner. Looks like this is about as good as it's going to get.
 
Fair enough. I was thinking maybe it would squeak just a bit more if the driver wasn't working into that load, but I guess not.
 
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